Many industrial processes, for example, chemical plants, petroleum refineries, bio-refineries, pulp and paper mills, produce large amounts of waste heat, i.e. heat that simply passes out of flue and stack gases, vent gases and combustion gases into the atmosphere. Most of the waste streams are liquid, gaseous, or a combination of both and have temperatures from slightly above ambient temperature to over 1100° C.
Capturing and reusing waste heat is an effective way to improve the overall energy efficiency of industrial processes. Typical heat recovery devices in industrial applications include recuperators, regenerators, economizers and waste heat boilers, all with heat exchangers. Although waste heat recovery technologies have already been employed in many industrial facilities to varying degrees, there exist technical and economic barriers which impede their wider application.
One of the less exploited waste heat resources is the low-temperature exhaust streams. It is estimated that about 60% of unrecovered waste heat has low quality, i.e., at temperatures below about 230° C. Although low-temperature waste heat has less thermal and economic value than high-temperature heat, it is ubiquitous and available in large quantities. Therefore, the total work potential of low-temperature waste heat is large and exceeds that of medium- and high-temperature waste heat together.
However, low-temperature waste heat is rarely recovered because exhaust streams need to be cooled below condensing temperatures to effectively recover both the sensible and latent heat, which causes severe corrosion problems on the heat recovery devices. Corrosive acids (e.g. H2SO4) with pH of about 2.2 and concentration as high as about 85% may form when dirty exhaust streams condense at temperatures below the acid and water vapour dew points. Heat exchangers made from low-cost materials (e.g. carbon steels) or even stainless steels, nickel-base alloys, etc., fail quickly due to the chemical attack on the heat exchanger surfaces. The high cost of exotic metals that can withstand the corrosive environments (e.g. tantalum, niobium, zirconium, titanium etc.) often prevents the economic employment of such devices for low-temperature waste heat recovery. Even the concept of forming a thin layer of the above-mentioned corrosion-resistant metals onto ferrous metals requires expensive and complicated coating techniques, which are not economically viable.
Combining the high corrosion resistance of polymers and the low-cost, high-strength and good thermal conductivity of steels is an intuitive approach to solving the problem. Many polymer coatings (e.g. phenolic and epoxy) have been applied onto metallic materials as barriers to protect the metals from chemical attack. However, in addition to their low thermal conductivity, these coatings have low application temperatures (below about 150° C.) and cannot be applied in higher temperature environments.
An existing technique was to utilize Teflon (polytetrafluoroethylene or PTFE) film to cover all heat exchanger surfaces contacting the flue gas and protect the heat exchanger from corrosion. PTFE is a fluoropolymer with excellent chemical inertness and a high application temperature of about 260° C., suitable for condensing heat exchangers in a relatively high temperature range. However, due to its extremely high viscosity after melting, PTFE would not flow at all and is thus not melt-processable, making it difficult to produce a pinhole-free protective layer with conventional coating methods.
As a result, although with excellent corrosion resistance, Teflon-covered heat exchangers have several disadvantages:                1) Teflon has very low thermal conductivity (˜0.2 Wm−1K−1 as compared to ˜400 Wm−1K−1 of copper), while the thickness of the Teflon film has to be >˜0.4 mm (which is about half of the thickness of typical heat exchanger tubes) to avoid pinholes. Thus the heat transfer coefficient of the Teflon covered tubes is very low.        2) The covering film does not have a physical bond with the substrate tube, thus very high heat transfer resistance exists at the Teflon/tube interface, further reducing the heat transfer efficiency.        3) Only bare tubes can be covered by Teflon films, the result is that the total heat transfer area is limited and the overall heat recovery efficiency of Teflon-covered heat exchanger is very low.        4) Since a large amount of costly Teflon PTFE will be used in the heat exchanger and the process to manufacture the PTFE-covered heat exchanger is expensive, the overall cost of the condensing heat exchanger is high.        
Therefore, there remains a need for a heat exchanger that can withstand the conditions of a condensing environment and with good heat transfer efficiency, adequate strength, minimum thickness and low manufacturing costs.